The long-term objectives of these studies have been to elucidate the mechanisms by which systemic hormones and localgrowth factors regulate the normal development of the mammary gland and milk protein gene expression, and to determine how these regulatory mechanisms have deviated in breast cancer. The 1998 National Cancer Institute Breast Cancer Program Review Group Summary Statement entitled, Charting the Course: Priorities for Breast Cancer Research" emphasized that "Our understanding of the biology and developmental genetics of the normal mammary gland is a barrier to progress...a more complete understanding of the normal mammary gland at each stage of development....will be a critical underpinning of continued advances in detecting, preventing and treating breast cancer." Our studies combine mouse genetics using transgenic, conditional knockout, and knockout transplantation models, with the in situ analysis of signal transduction pathways and complementary mechanistic studies in 3D cultures. These studies will continue to focus on the understanding of cell fate determination in the murine mammary gland with emphasis on the mechanisms regulating stem/progenitor cell self-renewal, normal ductal morphogenesis, and early progression in breast cancer. To accomplish these goals we propose the following specific aims: 1. To elucidate the role of the bZIP transcription factor C/EBPB in mammary epithelial cell fate determination: a. By comparing the effects of germline and somatic deletion of C/EBPB, the latter studies using a conditional allele for C/EBPB. b. By the analysis of gene expression in stem/progenitor cells in the wildtype and knockout mice as well as hormone receptor patterning and paracrine/juxtacrine mechanisms of proliferation, c. By using a C/EBPft-driven regulatable Cre for lineage tracking experiments. 2. To elucidate the molecular mechanisms of FGFR 1 and FGFR2 action in both 3D mammary epithelial cell cultures and in our previously described transgenic mouse model employing the chemically-inducible dimerization system, as well as in mice with a conditional FGFR2lllb allele. a. To study the role of the inflammatory process on early breast cancer progression, b. To elucidate the differences in FGFR1 and FGFR2 signaling with respect to effects on cell polarity, cell survival, invasion and migration. 3. To elucidate the role of the canonical and non-canonical Wnt pathways in mammary stem/progenitor self- renewal and survival. 4. To continue studies of the roles of C/EBPB, StatS and GR, and other co-modulatory factors in hormonally regulated chromatin remodeling at the B-casein promoter and enhancer. 5. To elucidate the role of Chk1 in normal mitosis and genomic instability in breast cancer.